The present invention relates to a safety photoelectric barrier for monitoring a protective field and to a corresponding method of monitoring the protective field.
Photoelectric barriers which are formed by one or more radiation beams are usually also referred to as light curtains, light grids or light barriers. Safety photoelectric barriers detect the movement or intrusion of objects, in particular persons, into guarded zones and may provide protection for human operators who are working with machines or other industrial equipment. Other light curtain systems are designed to control and monitor industrial automation processes and are in particular used for verifying assembly processes, counting objects, confirming an ejection process, recognize leading edges of transported goods, recognizing irregular shapes and a lot of other applications.
Safety photoelectric barriers employing infrared or visible light beams are used to provide operator safety by perimeter access control (PAC) in a variety of industrial applications. In particular, the operator protection around machinery, such as punch presses, guillotines, molding machines, automatic assembly equipment, coil winding machinery, robot operation, casting operations and the like can be ensured by using photoelectric barriers. Conventional light curtains typically employ light emitting diodes (LED) mounted at spaced positions along a transmitter bar at one side of the guarded zone, and phototransistors (PT), photodiodes or other photo receivers mounted along a receiver bar at the opposite side of the zone. The LEDs transmit modulated radiation beams along separate parallel channels to the PTs at the receiver bar. If one or more beams are blocked from penetration by an opaque object, such as the operator's body, the control circuit generates a safety signal that shuts down the machine, prevents the machine from cycling, or otherwise safeguards the area.
As mentioned above, such an interruption of the beam can also be used for counting objects or verifying the entrance of goods through defined areas.
It has to be noted in this context that the term “LED” sometimes is only intended to signify diodes that emit light in the visible spectrum, whereas diodes emitting infrared radiation are called IRED (infrared radiation emitting diode). However, in connection with the present invention, the term LED is intended to generally cover radiation emitting diodes irrespective of the wavelength of the emitted spectrum.
Usually, PAC photoelectric barriers comprise two active optical units, often called bars, sticks or strips, which are formed as two different constructional units, one of the optical units having the function of an emitter and one of a receiver. This architecture of an active emitter and receiver, however, has several drawbacks. Firstly, the fabrication costs are high, because both sides of the photoelectric barrier comprise expensive active components. Moreover, the installation is time consuming because the emitters and receivers have to be aligned accurately with respect to each other. Furthermore, both sides of an active-active photoelectric barrier have to be connected to the electric power supply and safety outputs.
In order to overcome these drawbacks, it is known from U.S. Pat. No. 7,034,950 B2 to form a laser grid by a single-sided arrangement in which the transmitting and receiving elements are combined into punctiform sensor units that each have at least one laser diode and two photosensitive pixels and that are arranged in line within a housing. According to this document, the distance of an intruding object is measured and a processing logic circuit is provided that is configured to logically combine the measured values of the plurality of the sensor units and to detect a size of an object from the combined values.
However, this conventional arrangement is not able to fulfill the safety requirements of safety integrity level 3 (SIL 3) as defined by the international standards IEC/EN 62061:2005, IEC/EN 61508:2010, and IEC/EN 61511:2003. In particular, IEC/EN 62061, “Safety of machinery: Functional safety of electrical, electronic and programmable electronic control systems,” is the machinery specific implementation of IEC/EN 61508. It provides requirements that are applicable to the system level design of all types of machinery safety-related electrical control systems and also for the design of non-complex subsystems or devices. The risk assessment results in a risk reduction strategy which in turn, identifies the need for safety-related control functions. These functions must be documented and must include a functional requirements specification and a safety integrity requirements specification.
The functional requirements include details like frequency of operation, required response time, operating modes, duty cycles, operating environment, and fault reaction functions. The safety integrity requirements are expressed in levels called safety integrity levels (SIL). Depending on the complexity of the system, some or all of the following elements must be considered to determine whether the system design meets the required SIL: Probability of Dangerous Failure per Hour (PFHD), Hardware Fault Tolerance, Safe Failure Fraction (SFF), Proof Test Interval (T1), Diagnostic Test Interval (T2), Susceptibility to Common Cause Failures (ß), and Diagnostic Coverage (DC).
There are four discrete safety integrity levels, SIL 4 being the highest and SIL 1 being the lowest level of safety integrity.
The problem underlying the present invention is to provide an improved safety photoelectric barrier that is economic to fabricate and easy to install, but on the other hand fulfills at least the requirements of safety integrity level 3.